Discriminator circuit



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DISCRIMINATOR CIRCUIT 2 Sheets-Sheet 2 Filed May 3, 1945 PAUL B., SEBRING, HARVEY BROOKS.

FEU. M

Patented Jan. 17, 1950 DISCRIMINATOR CIRCUIT Paul B. Sebring, Cambridge, Mass., and Harvey Brooks, Bennington, Vt.,

assignors to the United States of America as represented by the Secretary of the Navy Application May s, 1945, serial No. 591,760

s claims. (ci. 25o- 27) This invention relates to a discriminator, and

more specifically to an improved type of such device. The term discriminator, as used herein, signies a device for the conversion of variations in frequency to variations in amplitude. Such devices may be used to reproduce the modulation on a frequency modulated wave, for developing a direct current voltage responsive to changes in frequency of an incoming wave, or for other purposes. The discriminator hereinafter described may be advantageously used for the above or for any other purpose desired.

The present invention has for one of its objects to improve the operation of discriminators. More particularly, the general object is to pro- 'duce a discriminator with a much greater voltage output for a given change in frequency.

Other objects and advantages will become more apparent from the following detailed description of a preferred embodiment of the invention, in connection with the accompanying drawings in which:

Fig. 1 is a schematic circuit diagram of the invention;

Fig. 2 is a curve showing the characteristics of the filter used;

Fig. 3 is a curve of the operating characteristics of the invention; and

Figs. 4 1 1 show various vector diagrams of the i .operation of the device.

Referring particularly to Fig. 1, the discriminator includes a primary resonant circuit 3 inductively coupled to a secondary resonant circuit 4 5-6 which includes a center tapped inductance 4--5 and parallel capacitance 6, the two ends of which are connected to the anode and cathode elements of diode rectiers 1 and 8. It will be noted that rectiers 1 and 8 are connected with one end of inductance 4 to the anode of rectifier 1, and one end of inductance 5 to the cathode of rectifier 8.V

Connection between the resonant circuit 3 and input terminal is made, if desired, through ampli- -iier I8 ahead of lter unit l; and connection be'- tween the center tap of the resonant secondary -circuit 4-5-6 is made to the input terminals through amplifier I1 after filter unit I. The amplifiers I1 and I3 may be omitted if there is sufficient signal level at the input circuit.

The diode rectifiers 1 and 8 have suitable load resistors 9 and I0, which feed the direct current output voltage through resistors II and I2. The 'filter I and the tuned circuits 2, 3 and 4--5-6 are all tuned to the same frequency. When the input is at this frequency, equal and opposite voltages are set up across resistors 9 and I, and the direct current output is zero. When the input varies from the resonant frequency of the above circuits, one of the diode voltages increases while the other decreases, and a voltage equal to the difference between the -voltages across resistors 9 and I0 appears on the output lead. The sign of this voltage will of course depend upon whether the signal is greater or less than the above resonant frequency; and, if the variation is not too great, its magnitude will depend upon the disparity. Fig. 3 shows the characteristic curve of the discriminator with output voltage plotted against frequency.

The mode'of operation of the device is as described below.

Referring to Fig. 1, we see that this discriminator is essentially a phase-comparison system, sometimes called a phase sensitive detector, which involves the familiar principle of adding two alternating current voltages in such a fashion that a change in phase of one of them with respect to the other causes a magnitude difference between the two resultant voltages coming from the system. In this particular circuit, the phase Vchanges are the result of the conjoint action of a plurality of tuned circuits when the frequency of the input signal is varied.

The phase and magnitude with frequency characteristics of the voltages to be added will now be considered in order to see how the voltages, at the diodes, EA and EB, can change in magnitude with respect to one another.

Take first the voltage E1 which feeds the 'center-tap of the phase-comparison transformer I9. This voltage is affected by both the bridged- T filter I and the tuned circuit 2 which terminates amplifier I1. The locus of the output volt,- age of the bridged-T lter I with the input Voltage as a reference is approximately as shown in the vector diagram of Fig. 4.

The locus of the voltage vector in the tuned circuit is also approximately a circle, as shown in Fig. 5, when fed with an amplifier tube which acts as a series impedance.

As a result, the lo'cus of E1 with respect to the input voltage is another closed curve which, -though perhaps not exactly circular, may, for the purpose of our explanation, be considered as such. However, in the frequency spectrum from zero to infinity, we find that the end of this voltage vector traverses a curve which goes around It is evident that Ei+Ez=EA and E1-I-E3=EB,.

and what happens to E2 and E3 will nowA be considered. The primary and secondary coils of the phase-comparison transformer I9 are overcoupled to a large degree. As a result of this fact, it is found that the voltage across the'whole secondary (Ez-l-Ea) has a frequency characteristie as in Fig. 8. E2 exhibits; a curve ofthe same type, but half as large, whereas E3. exhibits a reversed image of this curve, since we refer both quencies, an additional audio frequency by-pass condenser may be used as a filter on the output lead.

If frequency modulated waves are applied to the discriminator, and I5 and I6 have the proper capacitance, it is evident that the modulation on the waves Will appear across resistors 9 and I0, and may bev tapped off.

The foregoing description sets forth only the preferred embodiment, and various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

voltages to the center-tap of the transformer secondary, that is, E3 is 180 out of phase v'with Ez.

To show what happens to EA and EB with frequency, consider the vector diagram of the` various voltages at the resonant frequency, Fig. 8A.

E1=0 from its diagrams.

Ez and E3 are equal andi opposite, and at. 90 to the input voltage.

Thus the rectied outputs of diodes A and B are equal and opposite vin polarity, thus making the discriminator D. C. output :0.

Next, consider a frequency slightlyv above resonance, which is represented in Fig., 9.`

It is easily seen that EA is considerably greater than EB and that therefore the D. C. output is positive. Note also. that E1 adds to E2 and bucks E3, and that itis not in quadrature withl these voltages.

Next consider a frequency of 940 cycles per second. Referring to the drawing, we find the voltages in. positions approximately as. shown in Fig. l0. The direct current output here ap,- proaches a positive peak.

Next consider an increased. frequency still further beyond the positive peak and f'25l200 cycles per second. This is shown inFig. 11, and it is seen thatthough the magnitudes of all vectors have decreased considerably, the magnitude of EA is still greater than that of EB s o that the direct current output is still positive.

For frequencies. below 300 cyclesl per second the operation is just the same except` that all vectors rotate in the opposite. direction as frequency decreases.

It is evident from Fig. 3 that this device has a very highA sensitivity at crossover, and this is due primarily to the fact that the, voltage from the bridged-T filter I goes through a phase reversal at resonance and acts either to aid E2V and buck E3 or vice, versa. Good sensitivity and wide range are both results of the fact that E1 is rotated not only by bridged-T filter I but additionally with the help of tuned circuit 2 so as to remain approximately in phase with either Ez or E3, depending upon whether the frequency of input is above or below resonance. The phase reversal characteristic of the lter I' determines whether E1 is in phase with E2 or E3, according to whether input frequency isabove or belowf'resonance.

The invention may be used' to controlan oscillator through a reactance; tube, or for other p urposes,` as desired. Furthermore the condensers I5 and I6' may be. either audio-or radio frequency by-pass condensers. If designed; for radio1 fre- What is claimed is:

l. A frequency discriminator circuit comprising a primary circuit resonant at a predetermined frequency and adapted to receive a signal voltage of variable frequency, a center-tapped secondary circuit resonant at said predetermined frequency inductively related to said primary circuit to produce equal and opposite voltages in the halves of the secondary circuit whose phase with respect to the signal voltage vary with changes in signal voltage frequency, a resonant bridged-T lter connected to said primary circuit and adjusted to produce a voltage and phase variation in the output voltage thereof proportional to variations in sig-nal voltage frequency with respect to the signal voltage, an impedance. connected across the out,- put of said bridged-T filter, means connecting one end of said impedance to the. center-tap of said secondary circuit, rectifying` means to rectify the resultant-voltage. between the other end of said impedance and each of the end terminals of said secondary circuit, and means to produce an output voltage proportional to the difference between the two direct current voltages produced by said rectifying means.

2. A frequency discriminator circuit including, a primary circuit resonant at a predetermined frequency and adapted to receive a signal volt'- age of variable frequency, aI secondary circuit resonant at said predetermined frequency inductively related to said primary circuit, said secondary circuit being center-tapped to provide equal and opposite voltages Whose phase with respect to said signal voltage vary with changes in frequency of the signal voltage, a resonant bridged-T filter connected to receive said signal voltage from said primary circuit, said bridged-T filter adjusted to cause the voltage and phase of the output voltage of the filter with respect to the signal voltage to vary with the frequency of the signal voltage, means to apply the output voltage of said bridged-T lter between the center-tap of secondary circuit and a point of fixed potential', means for rectifying the resultant potentials at the two ends of thesecondary circuit with respect to said point of fixed' potential, and means for producing an output voltage proportional to the difference between the twol rectified voltages produced by the rectifying means.

3. A frequency discriminator circuity comprising; a transformer having a primary coil paralleled by capacitance resonant at a predetermined frequency, a center-tapped secondary' coil in'- ductively relatedA to said? primary coil and paralleledV with capacitance to resonate at saidl predeterminedV frequency; said primary coil being adapted to receive an input signal of variable frequency, whereby saidsecondary coil produces avoltage between the7 center-tap and one end' of said secondary coilwhich is equal and opposite to the voltage between the center-tap and the other. end, of said secondary' coil and whose phase angle with respect to said input signal voltage varies with variations in input signal frequency; a resonant bridged-T filter circuit connected to said primary coil to also receive said input signal voltage and comprising a center-tapped inductance paralleled by capacitance to resonate at said predetermined frequency and an impedance having one end connected to said center-tap of said inductance, means connecting one end terminal of said inductance coil to one side of said primary circuit and the other end of said impedance to the other side of said primary circuit; a parallel resonant circuit comprising an inductance paralleled by capacitance to resonate at said predetermined frequency, means to connect said resonant circuit to the other end of said centertapped inductance of said bridged-T lter and to said other end of said impedance, whereby the voltage produced across said resonant circuit varies in phase and magnitude with respect to said input signal voltage with variations in said input signal frequency; means to apply the Voltage produced across said resonant circuit between said center-tap of said secondary circuit and ground and a second impedance connected between said center-tap of said secondary circuit and ground; a first rectifying means having a plate and a cathode and a second rectifying means having a plate and a cathode, means for connecting the anode of said first rectifying means to one end terminal of said secondary circuit and the cathode of said second rectifying means to the other end terminal of said secondary circuit, a first center-tapped resistance having its center-tap connected to ground, one end of said first center-tapped resistance being connected to the cathode of said rst rectifying means and the other end of said rst center-tapped resistance being connected to the anode of said second rectifying means and a second center-tapped resistance connected across said rst centertapped resistance, whereby the output of the discriminator circuit is the voltage between the center-tap of said second center-tapped impedance and ground.

PAUL B. SEBRING.

HARVEY BROOKS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

